Dispersion correcting antenna feed



ig am; I SEARCHROOM April 21, 1970 I A. c. SCHELL 3,508,273

DISPERS ION CORRECTING ANTENNAFEED Filed March 14, 1967 2 Sheets-Sheet 1INVENTOR. MA 6! GUI/44 I vBYZiggy;{ i g 2 April 21,1970 A. c. SCHELLDISPERSION CORRECTING ANTENNA \FEED 2 Sheets-Sheet 2 Filed March 14,1967 4 J .4 r m A, M MM am mawfwm United States Patent 3,508,273DISPERSION CORRECTING ANTENNA FEED Allan C. Schell, Winchester, Mass.,assignor to the United States of America as represented by the Secretaryof the Air Force Filed Mar. 14, 1967, Ser. No. 623,523 Int. Cl. H01q19/06 us. or. 343-754 3 Claims ABSTRACT OF THE DISCLOSURE The presentinvention relates to antenna feeds and more particularly to a feedutilized with a specialized antenna of multiple zones wherein thefrequency dispersive properties of zoned antennas are compensated for bya dispersion correcting feed.

Inherently zoned antennas have frequency dispersion limitations thusmaking them narrow band devices. This frequency dispersion occurs, forexample, because electromagnetic rays emanating from the feed to thereflector and thence in the desired direction do not have equalelectrical path lengths (inherent in the design). The conventional zonedantenna is designed so that the path length to the focus has been chosensuch that there is coherent addition of a received wave at a specifiedfrequency. With a shift in the specified frequency, the unequal pathlength from the wave front to the focus destroys the in phaserelationship of the received wave. The present invention provides meansto retain the in phase relationship over a wider frequency bandwidth bycompensating for the unequal path lengths.

The present invention provides a device that corrects for the frequencydispersive properties of zoned antennas, such as the multiple reflectortype of antenna. It may be utilized as a feed, or primary illuminator,for very large antennas. The device consists of a line feed or a planararray in which the individual radiating elements are interconnected bytransmission lines whose lengths have been chosen to compensate for theexternal path length differences across the antenna aperture.

The novel feature of the present invention is the use of a prearrangeddispersive characteristic of an extended feed to correct for thedispersive character of an antenna. Feeds have previously been builtwith special illumination patterns and means for removing aberrations,but this invention provides means for compensating for frequencycharacteristics over a band. It is to be noted that antennas thatconsist of zones, whether reflective or refractive, are inherentlylimited in their frequency range when operated with a conventional feed.This invention can greatly widen the frequency range of a zoned antennaby compensating for dispersion.

An object of the present invention is to provide an antenna having aprearranged dispersive characteristic to correct for the dispersivecharacteristic of a zoned antenna.

Another object of the present invention is to provide an antenna feed inwhich the individual radiating elements are interconnected bytransmission lines whose lengths have been chosen to compensate for theexternal path length differences across the antenna aperture.

3,508,273 Patented Apr. 21, 1970 These and other objects and advantagesof the present invention will be better understood when considered withthe following description taken in connection with the accompanyingdrawings made a part of this specification wherein embodiments areillustrated by way of example. The device of the present invention is byno means limited to the specific embodiments illustrated in the drawingssince it is shown merely for purposes of description.

In the drawings:

FIGURE 1 illustrates in schematic form the feed of the present inventionand the associated zoned reflector;

FIGURE 2 illustrates one embodiment of the feed of the present inventionincluding dipoles and associated preselected balanced transmissionlines; and

FIGURE 3 shows another embodiment of the feed of the present inventionshowing slotted waveguides.

Now referring to FIGURE 1, there is shown a schematic of a zonedreflector 10 and its associated feed 11. A zoned reflector of this typeis conventional and is described in IRE Trans on Antennas andPropagation, vol. AP-6, pp. l29133, January 1958, by L. Ronchi and G.Toraldo di Francia. It is also described in IRE Trans on Antennas andPropagation, vol. AP-9, pp. 48, January 1961, by L. Ronchi, V. Russo,and G. Toraldo di Francia.

Feed 11 and its associated transmission line 12 which comprises thepresent invention are shown in this instance, for purposes of a detailedexposition of its theory and mode of operation. Feed 11 is a line sourceof length L, consisting of a number of radiators connected by largeelectrical path lengths by means of transmission line 12, that isaligned vertically with its base at a height h above the aperture plane,as shown in FIGURE 1. Zoned reflector 10 extends outward from the pointunderneath the feed for a distance R =h tan a There is also shownpreselected antenna 13. The bottom section of the line source feeds thefarthest part of the reflector as shown by line 15, while the top of theline source illuminates the near part as shown by line 16. Theelectrical path length correction in feed 11 that is needed for raysleaving feed 11 at an angle a with the downward vertical is where z ismeasured along the vertical line feed and a and on are shown in FIGURE1.

The phase difference between adjacent radiating elements of the linefeed determines the direction of the maximum of the radiation from theelements by the relation The desired phase difference between radiatingelements can be obtained by appropriate taps on the path lengthsections. In general, the transmission line connecting any two elementsis longer than the straightline distance between elements, and thus thecondition for correction by a conventional line feed,

does not hold.

Since the line feed contains large path lengths, the phase differencebetween radiators shifts rapidly with frequency, changing the directionof radiation of the sections of the feed. If the line feed is to correctfor disper sion, the variation of path length along the feed must bechosen so that the phase difference between points in the aperture isapproximately constant with frequency.

The change of direction versus frequency of the radiation from a sectionof the feed is fdD 50!- dz csco: (2)

At the frequency f, a section of the line feed at z illuminates plate 14at an angle a; at the frequency f+6f, these plates are illuminated by afeed section located at z+6z. This feed section has a path length thatdiffers from that at z by dD EZBZ and a path length to the plates thatis greater by cos 1x52. If the relative phase across aperture 13 is toremain unchanged with frequency, then must be independent of Assuming hL, and using the approximation the relative phase dependence on itbecomes dD dz dz 2 dd a+cos a m I: h see a 5111 a-l-cos a 50:Substituting (2) and requiring independence of a,

dz da 2 2 l: it see a sin a+cosa :|sec a K The relation between z and ais found by integrating (3) to yield K log (K-sec 001e,,

The constant 2 can be determined from the property Z=O when a=a,,,yielding K log Elie tan a see a dd 16 5 integrating (5) yields 3 3 z-(sec 01 sec at) and applying the condition a=0 when z=L,

h K- -(see 05 -1) Substituting (6) into 1) with the approximation h L,

Z 1/3 z s a D(z) h sec 01 h[see 0: (sec 01 1) (7) The path lengthvariation of the line feed given by (7) corrects for the aperture pathlength variation within the accuracy of the approximations (h+z)-h and Ksec a. A more involved analysis along the lines of the above would yielda more accurate expression. At frequencies other than the design centerfor the feed, the illumination pattern will be displaced in angle due tothe phase change with frequency. This can be minimized by using a longfeed, oriented at an angle other than vertical.

The theory and mode of operation of the present invention is alsodescribed in a paper by Allan C. Schell entitled The Multiplate Antenna,by A. C. Schell, published in IEEE Transactions on Antennas andPropagation, vol. AP-14, No. 5, pp. 550-560, September 1966.

It is emphasized that the dispersion correcting feed described forFIGURE 1 functions by using electrical path lengths in the feed tocompensate for differences in path lengths for rays from different partsof the aperture. In FIGURE 1, there is shown a feed for a groundreflector on a plane. It should be clear that the zoned reflector neednot be on a plane; it may follow any curved surface desired.

There are many physical forms that an embodiment of this invention maytake. However if they are to perform the described function, each mustcontain the two essential parts: first, a series of basic radiatingelements, such as, horns, waveguide slots, or dipoles, and second, aseries of electrical path length delays, such as, waveguide sections,loaded transmission lines, or appropriately spaced subreflectors.

FIGURE 2 shows one representative embodiment of the feed of the presentinvention wherein line source feed 11 of FIGURE 1 is shown as a set ofdipoles 20-27 along the Z axis. Each of the dipoles being a radiatingelement. Insulating structure 28 is utilized to hold dipoles 20-27.There is provided antenna feed terminals 29 to which is connectedbalanced transmission line 30. Associated with transmission line 30 is aseries of transmission line sections 31--37 which are representative oftransmission line 12 of FIGURE 1. The length of each transmission linesection is cut so that the electrical path length of the section obeysthe desired phasing law given by Equation 7.

It is to be noted that in one method of dispersion compensation, areference point such as terminals 29 may be selected. Then there may beselected the electrical path length from the reference to eachsuccessive radiator dipole element such that the electrical path lengthvariation along the Z axis obeys Equation 7.

Now referring to FIGURE 3 which shows a zoned antenna feed being anotherembodiment of the present invention. Structure 40 is shown and isutilized for supporting waveguide 41. Waveguide 41 includes terminatingload 42. There is provided antenna input 43. The radiating elements arelongitudinal offset slots 44-51 which are cut in the broad face ofwaveguide 41. Waveguide 41 has been wound in a manner such that theelectrical path through the guide from slot to slot provides the correctvariation of electrical phase along the line feed axis as specified byEquation 7.

While in accordance with the provisions of the statutes, there have beenillustrated and described some preferred forms of the invention. It willbe apparent to those skilled in the art that changes may be made in theform of the apparatus disclosed without departing from the spirit of theinvention as set forth in the appended claims, and that in some casescertain features of the invention may sometime be used withoutcorresponding use of other features.

What is claimed is:

1. In a zoned antenna having a zoned reflector with a preselectedaperture, a feed therefor of predetermined length L comprising a seriesof radiating elements aligned vertically, a common input for said seriesof radiating elements serving as a reference point, said reference pointbeing at a preselected height h from said zoned reflector, atransmission line section connected from said reference point to each ofsaid radiating elements, the electrical path length D from saidreference point to each successive radiating element being of adifferent preselected length to compensate for differences in electricalpath lengths from said preselected aperture to said radiating elementsin accordance with the equation Z 1/3 D(z) h sec a-h [S003 OIO-(SGG3015-1) 2. In a zoned antenna having a reflector with a preselectedaperture, a feed therefor as described in claim 1 wherein said radiatingelements consists of dipoles.

3. In a zoned antenna having a reflector with a preselected aperture, afeed therefor as described in claim 1 wherein said radiating elements iscomprised of a series of longitudinal offset slots cut in the broad sideof a waveguide, said waveguide being wound so that the electrical pathlength through the waveguide from slot to slot pro- References CitedUNITED STATES PATENTS 6/1962 Strumwasser et al. 343--771 12/1968 Wong343771 X 10 HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR.,Assistant Examiner US. Cl. X.R.

